As an electric power compensating device having an electric accumulator made of an electric double layer capacitor (EDLC), an instantaneous voltage drop compensating device or an uninterrupted power supply (UPS) is included.
Here, with reference to FIG. 12, an instantaneous voltage drop compensating device using an EDLC as an electric accumulator will be explained.
As shown in FIG. 12, electric power is supplied from a system power source 1 to a load 3 via a high-speed switch 2.
The high-speed switch 2 is in a turned-on state under the control of a control device 4 when the system power source 1 is in a normal state, but if an instantaneous drop occurs in the system power source 1, the high-speed switch 2 is turned off, and when the system power source 1 recovers from the instantaneous drop after the occurrence thereof and returns to the normal state, the high-speed switch 2 is returned to the turn-on state.
The AC current of the system power source 1 is measured by an AC current meter 5 and a measured AC current value is sent to the control device 4, and the AC voltage of the system power source 1 is measured by an AC voltage meter 6 and a measured AC voltage value is sent to the control device 4.
A side-by-side placed capacitor 7 is an electric accumulator comprising EDLCs. More specifically, the configuration of the array board of the side-by-side placed capacitor is such that a plurality of capacitor modules in which 11 EDLCs are connected in parallel with each other are connected, for example, in a three-parallel and three-series configuration via metal conducting wires or conductors.
FIG. 13 shows one example of the side-by-side placed capacitor, where the reference numerals 7a to 7i denote capacitor modules (each in which 11 EDLCs are connected in parallel with each other), the capacitor modules 7a to 7c are connected in parallel with each other, the capacitor modules 7d to 7f are connected in parallel with each other, and the capacitor modules 7g to 7i are connected in parallel with each other.
Then, the group of the capacitor modules 7a to 7c connected in parallel with each other, the group of the capacitor modules 7d to 7f connected in parallel with each other, and the group of the capacitor modules 7g to 7i connected in parallel with each other are connected in series with each other to form a three-parallel and three-series connection array board configuration.
Referring back to FIG. 12 to continue with the explanation, an electric power transformer (converter/inverter) 8 performs a converter action and an inverter action under the control of the control device 4. That is, when the system power source 1 is in the normal state, the electric power transformer 8 performs a converter action to charge the side-by-side placed capacitor 7, when charging is completed, it stops the converter action, and when an instantaneous drop occurs, it performs an inverter action to DC/AC-convert the electric power of the side-by-side placed capacitor 7 and feed it to the load 3.
The DC voltage of the side-by-side placed capacitor 7 is measured by a DC voltage meter 9 and a measured DC voltage value is sent to the control device 4, and DC current outputted from the side-by-side placed capacitor 7 is measured by a DC current meter 10 and a measured DC current value is sent to the control device 4.
Since the side-by-side placed capacitor 7 is not charged at all when the side-by-side placed capacitor 7 has been first installed at a place where it is to be used, or when the side-by-side placed capacitor 7 has been fully discharged for maintenance, a converter 11 and a preparatory charging step-up/down chopper 12 are disposed in order to charge the side-by-side placed capacitor 7, and the converter 11 and the preparatory charging step-up/down chopper 12 are used to preparatorily charge the side-by-side placed capacitor 7.
Incidentally, after preparatory charging is completed, the side-by-side placed capacitor 7 is charged by the electric power transformer 8.
The control device 4 has a preparatory charge control unit 4a, a capacitor control unit 4b, a high-speed switch control unit 4c, and an inverter control unit 4d. 
The preparatory charge control unit 4a controls actions of the converter 11 and the preparatory charging step-up/down chopper 12 when preparatory charging is performed, the capacitor control unit 4b controls a charging action of the electric power transformer 8 to the side-by-side placed capacitor 7, the high-speed switch control unit 4c controls an action to turn on/off the high-speed switch 2 according to the occurrence of/recovery from an instantaneous drop, and the inverter control unit 4d performs control for the converter action and the inverter action of the electric power transformer 8.
The example in FIG. 12 has been explained as the instantaneous voltage drop compensating device, but the uninterrupted power supply also has the same basic configuration.
Incidentally, the instantaneous voltage drop compensating device is designed such that the side-by-side placed capacitor 7 can output a rated electric power for a few seconds (a rated compensation time period), and the uninterrupted power supply is designed such that the side-by-side placed capacitor 7 can output a rated electric power for a few minutes (a rated compensation time period).
Since the instantaneous voltage drop compensating device or the uninterrupted power supply thus configured is used for a long time period, it is necessary to diagnose the remaining lifetime of EDLCs.
In order to satisfy such a requirement, a system for measuring/determining the state of a capacitor is disclosed in Japanese Patent No. 3562633 (Patent Literature 1), and systems for predicting the remaining lifetime using capacitance are disclosed in Japanese Patent No. 4011016 (Patent Literature 2) and Japanese Unexamined Patent Application Publication No. 2008-17691 (Patent Literature 3).